The present invention relates to two stage furnaces. Specifically, the field of the invention is that of controls for two stage furnaces.
Conventional one stage furnaces cycle on and off to maintain a desired level of heat within a building. In operation, a thermostat senses a predetermined deviation from the desired temperature and activates the furnace. The furnace heats air which is circulated throughout the building. When the thermostat senses that the indoor temperature has reached the desired temperature, the furnace is shut down.
Conventional two stage furnaces also cycle on and off to maintain a desired level of heat, but can provide a more uniform flow of heat with greater efficiency. One prior art system uses timers to activate the two furnace stages in a predetermined sequence, the timing sequence being permanently programmed or dynamically alterable. In another prior art system, the furnace provides the low stage when the temperature differential is relatively low, and the high stage is provided during periods when the differential is relatively high. Thus, the operation of the furnace tends to match the heat demand of the building. However, problems exist concerning the prior art two stage furnaces.
One significant disadvantage with the prior art two stage furnaces is that they require expensive microprocessors and associated circuitry. One of the largest components of the cost of a furnace control is the circuitry of the microprocessor, so minimizing the complexity of controller board greatly reduces the total cost. Prior art control systems typically require a sophisticated microprocessor and substantial amount of supporting circuitry such as ROM and RAM.
Another disadvantage with the prior art involves the arrangement of temperature and pressure switches. Such switches are tested by the microprocessor which then executes the appropriate corrective steps. However, this requires that the switches be checked by the microprocessor for errors, after which the microprocessor independently executes the appropriate corrective steps by operating other elements of the system. Only the microprocessor can interrupt operation, and it must rely on external connections to implement an interruption.
An additional disadvantage concerns the comfort level provided by the prior art furnaces. The cycling of the furnace often begins with a blast of relatively cold air from a high speed circulator which is undesirable for the comfort of the occupants. A more desirable outcome would involve having warm air circulated immediately after the circulator fan starts so the occupants of the building are provided optimal heating.
A further disadvantage relates to condensation in the heat exchangers. The heat exchangers generally take longer to heat up during the low stage, which allows corrosive moisture to accumulate in the heat exchangers while warming up. Such condensation can shorten the useful life of the heat exchangers.
What is needed is a control for a two stage furnace which minimizes the cost of the microprocessing circuitry, which provides for redundancy in checking the temperature and pressure switches, which provides for better levels of comfort, and which minimizes the condensation in the heat exchangers.